Not liking the wash rack makes it really tough to keep him clean. I would work with him any way I could to get him used to it. What's the set up like, is there a way you could lure him in just a little bit more each time you introduce him (which I would recommend doing often!)
I had a friend who introduced me to using betadine (a wash often used on wonds) to keep her paint mare's points extra white before a show. It would also be helpful for nasty urine/poop stains. Just a little bit of betadine scrubbed onto the spot (that's of course already been hosed down to loosen up dirt and get the coat wet) and left for about 30 seconds and then promtly rinsed off works wonders! You may even be able to try this with just a bucket of water as long as you can throughly rinse the betadine off.